The invention is directed to an optical system for guiding a read or sensing beam in a magneto-optical storage system. In particular, the optical system has means for creating a polarized read beam and directing it on the storage medium which creates a reflected beam and a beam splitter which is positioned in the path of the reflected beam to deflect the reflected beam from the path of the read beam as a deflected beam. The system includes detector means which are connected to first means for creating a focusing control signal, second means for creating a tracking control signal, and third means for generating a raw data signal, and optical means for directing the deflected beam onto the deflector means.
In a magneto-optical storage system, as known, a light beam emitted from a laser light source is focused onto a selected track of a rotating storage disk. This is true both for the writing operation as well as for the reading operation. The only difference, among other things, is that a higher light intensity is used during the writing operation than during the reading operation. A typical property of a magneto-optical storage is that two control signals for exacting the position of the optical system relative to the storage medium must also be derived from a reflected laser light beam, which is split from the beam path of the sensing light beam. This beam, which is split from the path, is utilized in addition, to determine the raw data signals from the reading and to create the two control signals. First the laser light beam must be guided into a defined position radially relative to the storage disk and must be held there. This is accomplished in a known way by a control system for the track selection and for track following. Secondly, the laser light beam must be focused onto a storage region within such a track at every point in time and a second control system is utilized, which is referred to as focusing system for maintaining a predetermined vertical distance between the surface of the storage disk and the optical system.
Optical systems for guiding the read beam, which fulfill these jobs are known. An example is disclosed in an article by Y. Togami et al "Amorphous GdCo disk for thermomagnetic recording", Journal of Applied Physics, Volume 53, No. 3, March 1982, pp. 2335-2337. Another example of a known system is disclosed in an article by Y. Nagao et al "Dyanamic Recording and Readout Characteristics of GdTbFe Using a Modified Magneto-Optical Disk Exerciser", Japanese Journal of Applied Physics, Volume 21, No. 8, August 1982, pp. L509-L511. A third example is disclosed by N. Imamura et al "Experimental Study on Magneto-Optical Disk Exerciser With the Laser Diode in Amorphus Magnetic Thin Films", Japanese Journal of Applied Physics, Volume 19, No. 11, December 1980, pp. L731-L734. FIG. 1 in each of these three articles respectively shows a block circuit diagram for a magneto-optical storage and an optical beam path in the scanning system. What these arrangements have in common is that the reflected laser beam is repeatedly split by beam splitters and these sub-beams are then imaged onto spatially separated photo detector surfaces. Raw data signals or respectively control signals for both focusing systems and track following systems are acquired from the electrical output signals emitted by the individual photo detector surfaces. This multiple beam splitting has the disadvantages of increasing the cost but also has a disadvantage because the scanning unit to be positioned in two directions with respect to the storage disk has a relatively great mass because of the multitude of optical elements and therefore, is more mechanically sluggish.
Over and above this, the two essential forms for the acquisition of the control signal of the focusing system which are usually referred to as astigmatic imaging principle or respectively as the principle of the critical angle may also be obtained from the last two of these three articles. In the article by N. Imamura et al, FIG. 2 illustrates an imaging error of a cylindrical lens which is employed in conjunction with a four-quadrant photo detector in order to identify a focusing error. The article by Y. Nagao et al explains how the control signal for the focusing system is acquired with the assistance of a prism placed in what is referred to as a critical angle in the beam path of the sub-beams that are being deflected or split from the sensing beam.